A conventional road bicycle ("road bike") is generally equipped with two chainrings on the crank and a freewheel containing five to eight gears or sprockets. A road bike freewheel includes sprockets which, in some instances, are only one gear tooth apart from each other, producing a relatively narrow range of gear ratios. A road bike is, in general, configured to be ridden at relatively high speeds on smooth surfaces.
Road bikes are, in many instances, equipped with shifting levers having relatively small take-up spool diameters that produce a relatively small control cable displacement per angular displacement of the spool. This relatively small cable displacement is used to laterally shift a rear derailleur from one sprocket to the next. Road bicycle derailleurs thus have a relatively high actuation ratio, generally defined as the amount of derailleur movement perpendicular to the planes Of the freewheel sprockets (or chainrings) per unit displacement of the derailleur control cable.
More recently, mountain bicycles ("mountain bikes") have been developed which are ridden on trails that are not at all smooth; the "technical" portions of these trails commonly include sharp inclines, large boulders and tree trunks. The mountain bike freewheel thus includes a sprocket set that has a wide range of gear ratios. Further, mountain bikes generally include a third chainring with, in many instances, the diameter of the smallest chainring being substantially different from that of the largest chainring.
The conventional road and mountain bikes both employ (i) a front derailleur to effectuate movement of the drive chain from one chainring to another and (ii) a rear derailleur to effectuate movement of the drive chain from one sprocket to another.
As illustrated in FIG. 1, the conventional front derailleur 70 is generally actuated by a cable system 86. The cable system 86 includes a derailleur control cable 88 which is commonly a Bowden type--that is, the cable 88 is contained within a sheath that terminates in a ferrule affixed to the bicycle frame 12. The cable end 87 is affixed to the derailleur cable clamp 71 disposed on the extended end 79 of linkage member 78.
Typically, the front derailleur 70 takes a parallelogram form, defined by points (i.e., pivots) A, B, C and D. The derailleur 70 is affixed to the bicycle frame 12 by conventional means, such as a clamp 72 and a derailleur mounting member 74. Inboard and outboard linkage members 76, 78 hingedly connect the mounting member 74 to the chain guide 80. The chain guide 80 guides the drive chain 48 onto the selected chainring 40 of the crank 20.
The conventional front derailleur has several major drawbacks. The most significant drawback is the variation in actuation ratio over the range of motion of the derailleur. The variation is caused by virtue of the parallelogram form and is, in most instances, significant. A need therefore exists for a linear derailleur having a substantially uniform actuation ratio over the entire range of motion.
A second drawback of a conventional front derailleur is the number of moving components that are required to position the drive chain. The components are often exposed to water, mud, grit and the like, which could cause jamming or seizing of the derailleur. Further, the alignment and interaction of each component is critical toward efficient operation of the derailleur. A need therefore exists for a derailleur that is simple in operation and has a minimum number of components.
It is therefore an object of the present invention to provide a linear derailleur having a substantially uniform actuation ratio over the range of motion of the derailleur.
It is another object of the invention to provide a derailleur that is simple in operation and has a minimum number of components.